An electrically conductive member to be incorporated in a charging member for charging an electrophotographic photoreceptor is required to exhibit electrical conductivity of 10.sup.3 to 10.sup.9 .OMEGA. as calculated in terms of resistivity (hereinafter determined by means of an electrode having an area of 1 cm.sup.2). In general, it is provided on a metallic shaft and its external surface as an electrically conductive layer such as electrically conductive rubber layer. In order to make the electrically conductive member fully functional as a charging member, it is considered preferred that its resistivity level be in the range of 10.sup.3 to 10.sup.9 .OMEGA. as defined above. If there occurs a locally low electrical conductivity, it causes a phenomenon of passage of excess current through defective portions on the photoreceptor, i.e., so-called pinhole leak, resulting in image defects. Therefore, in order to inhibit pinhole leak, the lower limit of the resistivity of the electrically conductive rubber layer is preferably in the range of 10.sup.6 to 10.sup.9 .OMEGA.. However, if the resistivity of the electrically conductive rubber layer exceeds 10.sup.9 .OMEGA., no discharge occurs. Thus, the charged potential of the photoreceptor is not sufficient, causing the image to be entirely fogged (i.e., ghost).
In order to eliminate this defect, a function separation structure in which the resistivity of the electrically conductive rubber layer is kept low and a resin having a high resistivity is provided on the surface of the rubber layer as a resistive layer has been employed (JP-A-1-79958 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). However, this structure is disadvantageous in that different environmental conditions give different resistivities and hence different image densities. Another inevitable problem is that as the photoreceptor is repeatedly charged, the resin layer is unevenly worn, causing uneven discharging, or discharge products or part of toner constituent materials is attached to the surface of the resin layer, causing image defects. Further, the resin or rubber is worn while being brought into contact with the photoreceptor, producing rubber tailings that can be transferred to images.
The foregoing electrically conductive rubber layer is normally made of an electrically conductive rubber composition comprising a synthetic rubber such as EPDM rubber or silicone rubber with a powder of an electrically conductive material or electrically conductive fiber (carbon black, metallic powder, carbon fiber, etc.) incorporated therein. In order to allow the electrically conductive rubber layer to have an electrical conductivity as 10.sup.3 to 10.sup.9 .OMEGA., it is necessary that the powder of an electrically conductive material or electrically conductive fiber be uniformly dispersed within the plane. However, reproducibility and mass producibility problems such as resistivity variation within the plane and from lot to lot make it difficult to obtain sufficient properties.
In order to eliminate these difficulties, a charging roll has been proposed which is provided with an ionically conductive rubber layer that makes the use of the inherent ionic conductivity of synthetic rubber or an ionically conductive rubber layer obtained by adding a high dielectricity liquid or ionic substance to a synthetic rubber so that its ionic conductivity is increased (JP-A-2-199163). In this case, the ionically conductive rubber layer is a uniform dispersion system and thus can provide a uniform resistivity. However, it is disadvantageous in that when charging is repeated with a charging roll having the ionically conductive rubber layer being brought into direct contact with the surface of the photoreceptor, low molecular components contained in the rubber layer are transferred to the photoreceptor, causing image defects.
The charging roll is rotated while being pressed against the external surface of the exposing drum so that discharging occurs in the vicinity of the contact to charge the external surface of the exposing drum. Thus, the charging roll is required to be rigid enough to give no stress to the photoreptor. Thus, it is desired to replace the electrically conductive rubber layer by a rigid material such as semiconducting material. In this respect, a ceramic roller having a semiconducting substance free of elastic material is disclosed (JP-A-50-843). However, this ceramic roller can easily cause uneven discharging and thus cannot provide stable charging. This ceramic roller is also disadvantageous in that when it is rotated while being pressed against a photoreceptor made of a polymer material such as organic photoreceptor, it suffers from abrasion scratch or the like, resulting in the deterioration of the photoreceptor. Therefore, the state-of-the-art charging roll has an electrically conductive rubber roller.
As mentioned above, charging members such as charging roll which have heretofore been proposed all have problems in characteristics such as contamination on the photoreceptor, pinhole leak and deterioration due to the attachment of foreign matters or abrasion. Further, these charging members find difficulty in controlling the resistivity variation within the plane. Thus, these prior art charging members leave something to be desired.